Amphoteric surfactants are surfactants that contain both anionic hydrophilic groups and cationic hydrophilic groups in the same molecule. Their most significant characteristic is that they can both donate and accept protons. They have the following characteristics during use: Amphoteric surfactants generally have good performance in washing, dispersing, emulsifying, sterilizing, softening fibers, and antistatic, and can be used as fabric finishing auxiliaries, dyeing auxiliaries, calcium soap dispersants, dry-cleaning surfactants, and metal corrosion inhibitors, etc. They have excellent softness, smoothness, and antistatic properties for fabrics; certain bactericidal and mold-inhibiting properties; and good emulsifying and dispersing properties. However, this type of surfactant is relatively expensive, and its actual application range is smaller than that of other types of surfactants. It is a mild surfactant. Unlike single anionic or cationic surfactants, amphoteric surfactant molecules have both acidic and basic groups at one end of the molecule. The acidic groups are mostly carboxyl, sulfonic acid, or phosphate groups, while the basic groups are amino or quaternary ammonium groups. They can be mixed with anionic and nonionic surfactants and are resistant to acids, alkalis, salts, and alkaline earth metal salts.
Currently, commonly used synthetic amphoteric surfactants mostly have carboxylate groups as their anionic moieties, with a few having sulfonate groups; their cationic moieties are mostly amine salts or quaternary ammonium salts. Those with amine salts as the cationic moiety are called amino acid type; those with quaternary ammonium salts as the cationic moiety are called betaine type.
- Amino acid type
The aqueous solution of amino acid-type amphoteric surfactants is alkaline. When hydrochloric acid is slowly added with stirring, there is no change when the solution becomes neutral. A precipitate forms when the solution becomes slightly acidic. If more hydrochloric acid is added to make the solution strongly acidic, the precipitate dissolves again. This indicates that it behaves as an anionic surfactant in an alkaline environment and as a cationic surfactant in an acidic environment. However, when the cationic and anionic properties are exactly balanced at the isoelectric point, the hydrophilicity decreases, resulting in the formation of a precipitate. The anion in the molecule is a carboxyl group, and the cation is an ammonium salt. This type of surfactant exhibits different surface activities with changes in the pH of the medium. For example, dodecylaminopropionic acid (C12H25N+H2CH2CH2COO-) can be converted to sodium dodecylaminopropionate (C12H25NHCH2CH2COO-Na+) in a sodium hydroxide medium, behaving as a water-soluble anionic surfactant. In a hydrochloric acid medium, it can be converted to the hydrochloride salt of dodecylaminopropionic acid [(C12H25N+H2CH2CH2COOH)Cl-], behaving as a water-soluble cationic surfactant. If the pH of the medium is adjusted so that the positive and negative charges are exactly balanced, it is converted to an inner salt (C12H25N+H2CH2CH2COO-), which is poorly soluble in water and precipitates out. The pH at this point is called the isoelectric point. To fully exert the effect of amino acid-type amphoteric surfactants, they must be used in aqueous solutions with a pH deviating from the isoelectric point. Common raw materials used in the preparation of amino acid-type amphoteric surfactants include higher aliphatic primary amines, methyl acrylate (see acrylate esters), acrylonitrile, and chloroacetic acid.
- Betaine type
Betaine-type amphoteric surfactants have the biggest characteristic of being soluble in aqueous solutions whether they are acidic, neutral or alkaline. They do not precipitate even at the isoelectric point. In addition, they also have good properties such as permeability, detergency and antistatic performance. Therefore, they are good emulsifiers and softeners.
Carboxybetaine
The anion in the molecule is a carboxyl group, and the cation is a quaternary ammonium group. For example, alkyl dimethyl betaine [RN+(CH3)2CH2COO-], where the alkyl group R has 12 to 18 carbon atoms. Compared with amino acid types, betaine types can dissolve in water in acidic, neutral, or alkaline media, and do not precipitate even at the isoelectric point, so they can be used in aqueous solutions with any pH. In acidic media, when the pH of the isoelectric point is lower, it behaves as a water-soluble cationic surfactant [[RN+(CH3)2CH2COOH]Cl-]; in neutral or alkaline media, i.e., when the pH is equal to or higher than the isoelectric point, it behaves as a water-soluble amphoteric surfactant and does not behave as an anionic surfactant. Amphoteric surfactants tend to form precipitates with anionic surfactants only in acidic media. They can be used in combination with any type of surfactant in media with various pH values. Common raw materials used for preparing betaine-type amphoteric surfactants include alkyl dimethyl tertiary amines and sodium chloroacetate, etc.
Sulfobetaine
The anionic group in the molecule is the sulfo group (SO3-), and the cationic group is the quaternary ammonium group. Commonly used ones are alkyl dimethyl sulfonethyl betaines [RN+(CH3)2CH2CH2SO3-] and alkyl dimethyl sulfopropyl betaines [RN+(CH3)2CH2CH2CH2SO3-]. The number of carbon atoms in the alkyl group R in the formulas ranges from 12 to 18. Sulfobetaines have comprehensive properties; they not only possess all the advantages of ordinary betaines but also have unique advantages such as resistance to high concentrations of acids, alkalis, and salts. Currently, hydroxy sulfopropyl [RN+(CH3)2CH2CH(OH)CH2SO3-] is used to replace sulfopropyl betaines, which produce substances harmful to the human body during production. Due to the presence of both anionic and cationic groups with hydroxyl groups in their structure, they not only have all the advantages of amphoteric surfactants but also exhibit resistance to high concentrations of acids, alkalis, and salts, good emulsifying, dispersing, and antistatic properties, as well as bactericidal, antifungal, and viscoelastic properties. They are surfactants with excellent overall performance and have been widely applied in various fields such as daily chemicals, oilfield oil displacement, fracturing, and acidizing.
Phospholipid betaine
The anionic group in the molecule is the phosphate group (HPO4-), and the cationic group is the quaternary ammonium group. For example, alkyl dimethyl hydroxypropyl phosphate betaine [RN+(CH3)2CH2CH(OH)CH2HPO4-], where the alkyl group R has 12 to 18 carbon atoms. This structure determines that it not only possesses the excellent properties of amphoteric surfactants such as wettability, detergency, solubilization, emulsifying and dispersing properties, antistatic property, thermal stability, as well as good compatibility, low irritation, and advantages over general anionic surfactants such as better alkali resistance, electrolyte resistance, and antistatic property, but also has strong calcium soap dispersibility, low surface tension, and excellent foaming performance.
Post time: Feb-04-2026